A central feature of the immune response is protection from pathological responsiveness to self antigens. Dramatic progress in understanding self tolerance in T lymphocytes has been accomplished in the last few years. Recent work indicates that direct clonal anergy may occur in mature T cells in peripheral lymphoid tissues. This proposal is designed to investigate the molecular mechanisms responsible for maintaining clonal anergy in mature CD4+ T cells, using an in vitro system. Assays to determine if anergic cells are competent to recognize and bind antigen: APC will be done, and biochemical analysis of T cell receptor-associated molecules will be undertaken to define structural and functional changes. The proteins of interest include CD4, LFA-1, the tyrosine kinases p56lck and p59fyn and the tyrosine phosphatase CD45. Preliminary work has shown that p56lck expression is altered in anergic cells, and that it exhibits aberrant behavior after CD4 crosslinking, suggesting defects in CD4 signalling. Furthermore, antigen restimulation of anergic cells results in defective tyrosine phosphorylation of at least one cellular protein. These are the first changes in signalling mechanisms to be detected in anergic cells. The proposed experiments should provide important new information on the maintenance of anergy. This information is essential for the design of effective therapeutic protocols applicable to the treatment of immune dysfunctions, such as autoimmune disease, transplantation of allogeneic tissues, and the rejection of neoplastic cells. These studies should aid in providing further direction for the development of antigen-specific immune modulation.